1. Field of the Invention
The present invention relates to a valve control system and a valve control method, and more particularly to a valve control system and a valve control method that can meet safety standards.
2. Description of the Background Art
FIG. 13 illustrates an example of a conventional general-purpose valve. Referring to FIG. 13, a valve 101 is arranged in a pipe 1 through which a fluid (e.g., air, water, oil or the like) flows. Valve 101 can be opened by an electric signal, and specifically is a solenoid valve.
A spring 104 pushes a spool 102 so that valve 101 is normally closed. When the electric signal turns on a solenoid 103, solenoid 103 moves spool 102. Thereby, valve 101 opens. When solenoid 103 is turned off, a force of spring 104 returns spool 102 to an original position. Thereby, valve 101 closes.
In addition to the above general-purpose valve, valves meeting the safety standards have been provided (see “ISO valves in accordance with 5599-1 with position sensing of the piston spool”, “Safety engineering guidelines”, p62, [online], FESTO, [searched on Nov. 12, 2009], see Internet <URL:http://www.festo.com/net/fi_fi/downloads/Download.ashx?Ink=29273/HB_Safet y_en.pdf>). FIG. 14 illustrates an example of a valve meeting the safety standards.
Referring to FIG. 14, components that are the same as or correspond to those of the general-purpose valve shown in FIG. 13 bear the same reference numbers. A valve 110 complying with the safety standards (e.g., ISO 13849-1) is provided with a limit switch 115 for detecting opening/closing of valve 110.
When solenoid 103 is turned on, spool 102 turns on limit switch 115. Conversely, when solenoid 103 is off, spring 104 returns spool 102 to turn off limit switch 115. According to the structure shown in FIG. 14, the limit switch can detect the open and closed states of valve 110.
FIG. 15 illustrates an example of a structure of a conventional control system complying with the safety standards. Referring to FIG. 15, a valve control system 150 includes valves 110A and 110B as well as a safety controller 120.
Valves 110A and 110B are safety-standard-meeting valves and arranged in series in pipe 1. Each of valves 110A and 110B has the same structure as valve 110 shown in FIG. 14. A limit switch 115A detects the opening and closing of valve 110A, and a limit switch 115B detects the opening and closing of valve 110B.
Safety controller 120 receives signals in1 and in2 from limit switches 115A and 115B, respectively, and provides signals out1 and out2 for controlling solenoids 103A and 103B, respectively.
Safety controller 120 controls valves 110A and 110B as described below. First, when safety controller 120 simultaneously turns on signals out1 and out2 so that solenoids 103A and 103B are turned on, respectively.
Then, safety controller 120 checks that both signals in1 and in2 are on. When both signals in1 and in2 are on, both valves 110A and 110B are open. Therefore, safety controller 120 continues the on states of signals out1 and out2.
Valves 110A and 110B, limit switches 115A and 115B, and safety controller 120 are components satisfying the safety standards. Valve control system 150 shown in FIG. 15 can satisfy the predetermined safety standards.
The above safety standards are, e.g., ISO 13849-1. The structure shown in FIG. 15 can comply with a category 4 defined by ISO 13849-1.
When one studies safety protection in measures for reducing a risk of a machine, estimation of a magnitude of the risk as well as a performance reference of the safety system corresponding to it has been generally represented by the “category” in the European standards EN 954-1 or the international standards ISO 13849-1:1999 based on it. The “category” is an architecture of the safety control system, and is based on a so-called definite technique provided by electro-mechanism parts such as a contact-point technique of switches and relays that have been built up.
FIG. 16 illustrates categories defined by ISO 13849-1:1999. Referring to FIG. 16, ISO 13849-1:1999 defines five categories “B”, “1”, “2”, “3” and “4”. As the category changes from “B” toward “4”, the achievement level of the performance reference increases.
The revised edition of ISO 13849-1 defines five indexes from “a” to “e” that are called “PL (Performance Levels)” as the indexes of evaluation of the safety control system. The PL has taken concepts of “reliability” and “quality” into the conventional concept of the “category”, and evaluates the Mean Time To Dangerous Failure (MTTFd), DCavg (Average Diagnostic Coverage) and Common Cause Failure (CCF). The PL can quantitatively evaluate the safety control system according to a state of actual use.
The official name of the revised edition of ISO 13849-1 is “ISO 13894-1 (Second edition Nov. 1, 2006) Safety of machinery Safety-related parts of control systems, Part 1: General principles for design). In the following description, the revised edition of ISO 13894-1 will be described as “ISO 13849-1:2006”.
According to ISO 13849-1:2006, important matters of the safety control system required in each category are the same as those of ISO 13849-1:1999 or EN 954-1. However, each safety control system is diagrammatized by handling three parts, i.e., I (Input unit), L (Logic unit) and O (Output unit) as axes for clearly showing respective features.
FIG. 17 is a block diagram for illustrating important matters of the safety control system required in each category represented by ISO 13849-1:2006.
Referring to FIG. 17, the structures applied to the categories B and 1 can be implemented by I, L and O. The structure applied to category 2 can be implemented by adding a TE (TEst unit) to the above I, L and O. The structure applied to the categories 3 and 4 can be implemented by duplexing the above I, L and O. The category 4 differs from the category 3 in that it requires a higher detection capability than the category 3, but employs the same structure as the category 3.
FIG. 18 is a graph for illustrating a method of evaluating the performance level. Referring to FIG. 18, four parameters, i.e., a category (indicated as “Cat” in FIG. 18), MTTFd, DCavg and CCF are used for evaluating the PL.
As shown in FIG. 18, there is a plurality of combinations of the parameters that can achieve the performance level of, e.g., “c”. In other words, appropriate combinations of the foregoing four parameters can achieve the desired performance level. Therefore, it can be considered that ISO 13849-1:2006 achieves more flexibility in construction of the safety system than conventional ISO 13849-1:1999.
For example, a system in a factory uses the general-purpose valves shown in FIG. 13 in many portions. Conventionally, for constructing an existing system to comply with the safety standards (e.g., category 3 or 4 of ISO 13849-1), it is necessary to add the safety-standard-meeting valves to the system and further to duplex such safety-standard-meeting valves.
However, as can be seen from a comparison between FIGS. 13 and 14, the safety-standard-meeting valve is a special part that differs in structure from the general-purpose valve. For example, therefore, the following problems occur when the safety-standard-meeting valves are introduced into the existing system for constructing a system meeting the safety standards.
First, design of the system is complicated because the safety-standard-meeting valves that are the special parts are added to the existing system. Also, it becomes necessary to manage the stock of the safety-standard-meeting valves in addition to that of the general-purpose valves for operating the system. Therefore, the stock management becomes complicated, and the management for maintaining the system becomes complicated.
Since the safety-standard-meeting valves are special parts, choices or options thereof are fewer than those of the general-purpose valves. This restricts the system design. Since the safety-standard-meeting valve is generally more expensive than the general-purpose valve, the cost for constructing and maintaining the system may increase.
Further, the safety-standard-meeting valve has a switch for detecting the opening and closing thereof, and therefore has larger sizes than the general-purpose valve. When the safety-standard-meeting valve is introduced into the existing system, the scale of the system may increase. Therefore, the arrangement of the safety-standard-meeting valves significantly restricts the design of the system. Thus, the safety-standard-meeting valve restricts the flexibility in system design.
When the safety-standard-meeting valves are employed, the completely open state and the completely closed state can be checked by matching the stroke of the spool with the stroke of the limit switch. However, this structure results in a problem that a state (e.g., a slightly open state) intermediate between the completely open state and the completely closed state cannot be detected.
As described above, the conventional valve control system complying with the safety standards suffers from a problem that the structures and specifications of the safety-standard-meeting valves restrict the specifications of the system.